Method and apparatus for determining gas permeability of film

ABSTRACT

A METHOD AND APPARATUS FOR DETERMINING THE PERMABILITY OF MEMBRANES USING A MULTIPLICITY OF PERMEABLE CELLS. THE UPSTREAM COMPARTMENTS ARE CONNECTED IN SERIES AND ARE SUPPLIED WITH A CONTINUOUS STREAM OF PERMEABLE GAS. TWO VALVE MEANS ARE OPERATIVELY ASSOCIATED TO DIRECT CARRIER GAS THROUGH A SELECTED ONE OF THE DOWNSTREAM COMPARTMENTS AND THROUGH A DETECTOR AND SIMULTANEOUSLY, A PURGE GAS IS DIRECTED THROUGH THE NON-SELECTED CELLS BY SHIFTING THE OPERATIVE POSITIONS OF THE VALVES, PURGE GAS MAY BE DIRECTED THROUGH THE FORMERLY SELECTED COMPARTMENT WHILE CARRIER GAS IS DIRECTED THROUGH THE FORMERLY NON-SELECTED COMPARTMENT. MEANS FOR BYPASSING THE DOWNSTREAM COMPARTMENTS IS PROVIDED IN THE VALVES. EACH VALVE IS PREFERABLY OF A ROTARY TYPE AND INCLUDES SEALING MEANS FORMED OF AN ANNULAR GROOVE AROUND THE VALVE SWITCHING PORTS WITH A CONTINUOUS STREAM OF CARRIER GAS FLOWING THEREIN. IN A PREFERRED EMBODIMENT OF THE CELL ASSEMBLIES, THE MEMBRANE IS RETAINED AND SEALED BETWEEN THE UPSTREAM AND DOWNSTREAM COMPARTMENTS USING SPRINGS WHICH URGE THE MATING WALL OF ON E OF SAID COMPARTMENTS TOWARD THE OTHER COMPARTMENT MOUNTED ON A SPRING FLOATED BASE.

NOV. 9, 1971 STEPHENS ET AL 3,618,36fl

METHOD AND APPARATUS FOR DETERMINING GAS PERMEABILITY OF FILM 4 Sheets-Sheet 1 Filed March 2, 1970 INVENTORS Thomas M. Stephens y James A. McNu/ZSM 44, M m Attorneys T. M. STEPHENS E AL 3,6183% Nov. 9, 1971 METHOD AND APPARATUS FOR DETERMINING GAS PERMEAHILITY OF FILM 4 Sheets-Sheet 2 Filed March 2 1970 IN VENTO Thomas M. Stephens James A. McNcllr NOV. 9, 1971 STEPHENS ETAL 3,61,36H

METHOD AND APPARATUS FOR DETERMINING GAS PERMEABILITY OF FILM Filed March 2 1970 1 Sheets-Sheet 5 s n MFS mm? W W W ,m WSW m mM fi S A as me mm T@ Y B NOV. 9, 1971 STEPHENS ET AL 3,618,361

METHOD AND APPARATUS FOR DETERMINING GAS PERMEABILITY OF FILM 4 Sheets-Sheet 4 Filed March 2 1970 INVENTORS, Thomas M. Stephghs BY James A. McNuIry W M Attorneys United States Patent Ofice US. CI. 7338 9 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for determining the permability of membranes using a multiplicity of permeable cells. The upstream compartments are connected in series and are supplied with a continuous stream of permeable gas. Two valve means are operatively associated to direct carrier gas through a selected one of the downstream compartments and through a detector and simultaneously, a purge gas is directed through the non-selected cell-s by shifting the operative positions of the valves, purge gas may be directed through the formerly selected compartment while carrier gas is directed through the formerly non-selected compartment. Means for bypassing the downstream compartments is provided in the valves. Each valve is preferably of a rotary type and includes sealing means formed of an annular groove around the valve switching ports with a continuous stream of carrier gas flowing therein. In a preferred embodiment of the cell assemblies, the membrane is retained and sealed between the upstream and downstream compartments using springs which urge the mating wall of one of said compartments toward the other compartment mounted on a spring floated base.

It is known to use a single permeation cell wherein a membrane forms a partition between an upstream compartment and a downstream compartment. A permeant gas is fed continuously to the upstream compartment and simultaneously a carrier gas is directed through the downstream compartment, where it picks up any permeant gas that is diffused through the membrane, and the permeant-carrier gas mixture is directed to a detector for analysis. One such cell, employing substantially no pressure differential between compartments, is quite rapid and sensitive to the low rates of permeation. Consequently, this type of cell is particularly effective in measuring relatively impermeable membranes such as plastic films conventionally used in packaging.

Single permeation cell and detector apparatus of the above type can only perform one measurement at a time. For certain thin plastic films, attainment of the required steady state for detection frequently requires substantial periods of time to purge extraneous gases from the membrane and cell compartments. Consequently, where a large number of membranes are to be measured, measurement must be delayed until a steady state permeation rate occurs. A further disadvantage of typical configurations of the above described cells resides in the manner of clamping the membrane across the cell. This clamping is conventionally performed by using threaded member against the mating surface in a fixed rigid position. Such a technique frequently results in leakage due to a break caused thereby in the membrane or by a disproportionate tightening of one side with respect to the other thus raising the opposite side.

3,618,361 Patented Nov. 9, 1971 SUMMARY OF THE INVENTION AND OBJECTS It is an object of the invention to provide a method and apparatus for determining the permeation rate through a membrane by means of a plurality of cells having upstream and downstream compartments separated by the membrane retained therebetween.

It is a further object of the invention to provide an apparatus of the above type with an improved retention means for the membrane.

Other and further objects of the invention will be apparent from the following description in which the preferred embodiment is set forth in detail in conjunction with the accompanying drawings.

In accordance with the above objects, apparatus. is provided for determining membrane permeation coefficients rates through membrane including first and second permeation cells having upstream and downstream compartments adapted to be separated by the membrane retained therebetween, a conduit for permeant gas flow into said first cell upstream compartment, a passage between said first and second upstream compartments, and a vent for the latter compartment. A first control valve having different operating conditions is provided to direct carrier gas into either said first or second downstream compartment, hereinafter the selected compartment. A second control valve operatively associated with the first valve serves to direct the carrier and diffused permeant gas from the selected compartment through a detector. The first and second control valves are further operatively coupled to direct a purge gas through the nonselected downstream compartment for venting to the atmosphere. Two, three, four or more operatively coupled permeant cells are within the scope of the invention.

A rotary rubbing seal valve is particularly effective as the first and second control valves above described. To avoid leakage from the surroundings at the rubbing seal, we have provided an annular groove filled with fiowing carrier.

In another feature of the invention, a bypass is provided whereby carrier gas may be directed through the first and second valves thence to said detector means without flowing through any of the downstream compartments. This provides a convenient means for calibrating a reference base line for the detector.

An improved means for retaining the membrane between the upstream and downstream compartment is also provided. A spring means urges the mating wall of one of said compartment toward the other compart ment which is mounted on a spring floated base to provide a uniform force over the entire mating surface. This avoids a threaded clamping which is prone to leakage as above described.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic flow diagram of a system embodying the present invention.

FIG. 2 is a side elevational view, partly in section, of a cell assembly according to the invention.

FIG. 3 is a bottom view, partly broken away, of the assembly shown in FIG. 2.

FIG. 4 is an end elevational view of the cell assembly shown in FIG. 2.

FIG. 5 is a diagonal sectional view of FIG. 3 taken along the line 55.

FIG. 6 is a top view of FIG. 5 along the line 6-6.

FIG. 7 is a top view of FIG. taken along the line 77.

FIG. 8 is an exploded view of one embodiment of a valve according to the invention.

FIG. 9 is an assembled side view of the valve shown in FIG. 8.

FIG. 10 is a cross sectional view of the valve of FIG. 9 along the line 10-10.

FIG. 11 is a cross sectional view of the valve of FIG. 9 taken along the line 1111.

FIG. 12 is a cross sectional view of a portion of FIG. 11 taken along the line 1212.

FIG. 13 is a cross sectional view of FIG. 9 taken along the line 13-13.

FIG. 14 is a cross sectional view of a portion of FIG. 13 taken along the line 1414.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the flow diagram of FIG. 1, the invention will be most readily described by first illustrating the inventive device and method with a particular setting of valves and then demonstrating the changes affected by repositioning the valves. The flow system includes three permeation cells 21, 22, and 23 each including upstream compartments 24, 25, and 26 and downstream compartments 27, 28 and 29 the corresponding upstream and downstream compartments are separated by membranes 30. The system is further provided with rotary control valves 31 and 32. In one flow system, carrier gas, suitably helium, is split into two supply lines 33 and 34. Line 33 and a first and second permeant gas supply lines 35 and 36 are directed to a three way switch valve 37 which selects which of the three inlet gases is to flow through flow control valves 38 and via supply line 39, provided with a flow restricter and heat sink, into up stream compartment 24. In a first procedural step the entire system is purged with carrier gas and so 37 interconnects lines 33 and 39 and valve 38 is opened. Carrier gas then exits from compartment 24 and flows into compartment 25 via line 40 in like manner the same gas exits from compartment 25 into compartment 26 via interconnecting line 41 from whence the gas is vented to the atmosphere via exhaust line 42.

Valve 31 is shown in a position to supply carrier gas to downstream compartment 27 and thence through valve 32 to detector 43 while compartments 28 and 29 are supplied with a continuous purged gas to maintain these unused compartments at steady state as will be explained herein. It is noted that the arrows shown on the drawings indicate the direction of gas flow regardless of valve settings or type of gas flowing therein. Carrier gas flows through line 33 which bisects into line 46 provided with valve 47 and a flow restricter, and into line 48 provided with valve 49 a pressure gauge and a flow restricter. In the setting shown in the drawing carrier gas flows through open valve 49 into inlet to 50 of valve 31 and thence through outlet to 51 communicating therewith in this valve position through line 52 to the inlet side of compartment 27 and from thence via line 53 to inlet port 54 of valve 32 which communicates via channel 56 with outlet port 57 and, in turn, via line 58 through detector 43. In this valve arrangement any permeant gas which diffuses from compartment 21 into compartment 27 is swept from the latter compartment through detector 43. However, since with the valve settings as shown only carrier gas is flowing through the upstream compartments, a zero base line may be established with this setting.

Gas flowing through line 46 flows through an angular purged groove about compartment 27 to prevent leakage and flows therefrom via lines 59 and 60 to similar grooves and compartments 28 and 29. From the latter channel the purging carrier gas is vented to the atmosphere.

A system is supplied with a carrier gas flowing through line 61 and through line 64 and valve 66 which continually sweeps past any of the downstream chambers not being supplied with carrier gas flowing to the detector. Thus with the setting as shown in FIG. 1, compartments 28 and 29 are purged. Line 61 is bisected into a detector reference line 62 provided with a flow valve 63 and a flow restricter and a cell purge line '64 provided with a valve 66. Line 62 forms a detector reference stream for comparison with the carrier-permeant gas mixture to be measured. Carrier gas proceeds through line 64 into inlet tube 67 which communicates in a C-shaped groove 68 with outlet tube 69, 70 and 71 which in turn communicate respectively with lines 72, 73 and 74. Carrier gas is directed via line 73 provided with a heat sink into compartment 28 and therefrom via line 76 into C-shaped communication groove 77 of valve 32. In like manner, carrier gas is directed via line 72 provided with a heat sink into compartment 29 and therefrom via line 80 into inlet port 81 communicating with groove 77. Conduit 82 provides a path for carrier gas from groove 77 into annular groove seal 83 from whence the carrier gas travels via line 84 into a similar groove seal 86 and from thence the carrier gas is vented to the atmosphere via line 87.

In order to take a permeation measurement of gases 33 or 36, valve 37 is switched to the appropriate position and the permeant gas travels via line 39 and sweeps through compartments 24, 25 and 26 and thence to the atmosphere. A measurement is taken in detector 43 after the sweeping permeant gas has attained a steady state diffusion rate through the membrane. Alternatively, other measurements may be made such as rate of rise (slope) or diffusion at different cell temperatures.

A simplified manner of obtaining a zero reference base line by flow through line 58 through the detector involves the complete bypass of the downstream cell portions by the carrier gas. In this system, carrier gas flows through line 48 into valve 31 and then valve 32 and via line 58 through the detector. As with other changes in operating position of valves 31 and 32, the lines are maintained in a fixed position while the illustrated valve face, containing the radial and C-shaped groove is rotated to the appropriate position. To obtain the described bypass, the radial groove in valve 31 is rotated to communicate with line 74 which in turn communicates with the radio groove of valve 32 which directs the carrier gas via line 58 through the detector.

To obtain a permeation coefficient of membrane 30 in cell 22, the settings of valves 31 and 32 are coordinately changed. The groove of valve 31 is rotated clockwise to tube 70 communicating with line 73' while groove 56 of valve 32 is rotated counterclockwise to next successive opening communicating with line 76. In this manner with the valve arrangements as shown, carrier gas flows via line 48, valve 31, line 73, through compartment 28 and exits therefrom via line 76, valve 32, line 58 and through detector 43. By following the changes made by rotating the shown face of valves 31 and 32 it is apparent that carrier gas flows through cells 21 and 23 and vents into the atmosphere while carrier gas flows through cell 22 and sweeps permeant gas into detector 43.

In like manner, carrier gas may be made to flow through compartment 29 for permeation detection by rotating radio groove 56 of cell 31 clockwise to communicate with lines 72 and simultaneously rotating radio groove 56 counterclockwise is to communicate with line 80.

It is noted that in operation the purging gas flowing through the non-selected downstream compartments travels at a substantially greater flow rate than the carrier gas in the selected compartment.

The combination of needle valves and flow restrictions referred to above permit setting the carrier gas flow rate between 1 and 200 ml./rnin. A permeation cell as described herein accepts a membrane with an exposed area on the order of 10 cn1. and a thickness on the order of from below 0.5 mil upward. The cell volume is typically a fraction of one ml. Rapid leak-tight mounting of the membrane is assured by a unique construction described herein after employing a spring-loaded lever-fulcrum arrangement. The permeation cells are enclosed in a controlled temperature environment covering a range from below ambient to 150 C. or more.

Detector 43 may be an ultrasensitive katharometer circuit measuring changes in the gas stream composition caused by permeation through the membrane. The temperature of the katharometer is controlled separately and thus is completely insensitive to temperature changes of the permeation cell. Nominal sensitivity is 7x10- mv./p.p.m. carbon dioxide in helium. Zero drifts are less than 1.5 X 10- mv./hr. Carbon dioxide permeation rates of about 10* cc./cm. /sec. or 0.1 cc./ 100 in. /24 hr. can be detected at a flow rate of 3 cc./min. Further increase in sensitivity is feasible.

The absolute sensitivity of the katharometer was determined from the signal produced by a helium-carbon dioxide mixture of known composition. Calibration factors for the other gases were derived from the known molar sensitivity factors of the katharometer. Calibration factors for vapors were determined also by weighing methods.

In an alternative to the katharometer, a gas chromatograph may be used as detector 43.

A variety of permeants can be admitted to the upstream compartment. They may be noncondensible gases such as carbon dioxide, oxygen, nitrogen, vapors including water; and liquids. The gases and vapors may be pure or diluted with carrier gas in a defined ratio. The term gas as used in description of the permeant is intended to include gas, vapors and liquids.

The above multi-celled device is equally suited for research or quality control functions. The three cells enable separate determinations on a single strip of film. Alternately, three different pieces or types of film can be tested sequentially. The use of three cells also minimizes the time required for initial outgassing of the membrane.

Referring to FIGS. 2-7, the cell assembly includes the three cells 21, 22 and 23 disposed in and supported by a housing 90 and a camshaft assembly 92 rotatably mounted at opposite ends of the housing. Each of cells 21, 22 and 23 include an upper portion 24, 25 and 26 and a lower portion 27, 28 and 29. For simplicity, the remainder of this description will relate to cell 22 which is essentially the same as the other cells. The cell bottom 28 is held in position, suitably by cap screws as shown in FIG. 3, to the support plate 94 which applied a force through four springs 95. This has the effect of preloading cell bottom 28 with a moderate force. Upper portion 25 includes an integral stem which is free to move in a vertical direction through an opening in support plate 96. Spring 97 is provided to urge the top of portion 25 continuously toward the mating surface portion of cam 92. The spring is held in position by a retaining ring 98. A washer is provided between the cam surface and the top of portion 25 to provide a smooth sliding action. A handle portion 99 of camshaft 92 extends through the housing for ready rotation of the cam with a corresponding raising or lowering of upper portion 25 to open or close cell 22.

Both upper and lower portions 25 and 28 respectively are provided with internal chambers having circular lands 100 with broken grooves cut into the lands facing in opposite directions to direct a flow of gas. Upper portion 25 includes an inlet port passage 101 extending from the periphery thereof through the chamber where the entering gas flows around the periphery of the lands and through the grooves to exit from an outlet port passage 102 extending vertically from the center of the chamber through portion 25 and then radially outward therefrom. Portion 25 is also provided with an O ring 103 which sets into an annular groove to insure an optimum seal between the mating metal surfaces of upper and lower portions 25 and 28 at the membrane surface. Lands 100 also serve to support thin membranes in a generally flat configuration to prevent deformation or oscillation of the same during slight pressure differentials in the cell. Lands are slightly recessed below the seating surface for readly calculation of the exposed surface area in the cell.

The lower cell portion 28 is provided with an inlet port passage projecting axially into the outer portion of the chamber and an outlet port passage 104 projecting outwardly from the center of the chamber. In addition, an annular purge ring 106 is positioned on the lower portion and includes an inlet port 107 and an outlet port 108. An 0 ring 109 is positioned in a groove concentric with ring 106 and radially outward therefrom. A purge gas flowing through ring 106 prevents extraneous gas from the surroundings or from the O ring from entering the downstream chamber by purging the same gases.

As can be seen from FIG. 1, the purge gas lines to the three cells and the gas flowing into and out of the upstream compartments are arranged in series.

Means, not shown, are provided to both heat and cool the entire multiple cell assembly. The assembly is preferably disposed within an insulated enclosure. To assure that the gases entering the cell reach the equilibrium temperature of the cell chamber, means for heating the same gases are provided prior to their entering the cell chamber in the upstream portion of the first cell in series. Such heating is performed by forming the tubing fitting onto inlet 101 into a helical coil which may be rigidly attached to the side of the chamber as with a good heat-conducting material, such as silver solder. The downstream portions of all three cells are provided with heat sinks in the undercut portion 140 suitably by an annular helical coil of tubing communicating with the downstream chamber.

In operation, placement of the membrane in the above cell assembly is performed as follows: As shown in FIG. 2, the cells may be opened or closed depending on the rotation of the mating cam surface. It can be seen that by actuating the cam, portion 26 is urged against portion 29 and drives the latter portion downwardly against its supporting springs to effect a spring loaded seal. With this type of seal, a uniform spring force is applied across the entire mating surface since the floating springs accommodate slight variations in tolerances. It is further noted that the cam arrangement is such that all cells are opened and closed simultaneously.

Valve 31, identical to valve 32 except for external connecting pipe 82 in valve 32, is shown in FIGS. 8-14. Valve 31 includes a valve body 110 in which is disposed the operative rotary sealing surfaces formed of valve seat 111 urged against face plate 112. Seat 111 is urged by rotating and pressure producing spring assembly. This assembly includes a valve seat driver 114 with pins matching grooves in valve seat 111 which is urged toward the sealing surface of the valve by a train of springs, washers and bearings generally designated 116 held under compression in body 110 by retaining rings 117. Shaft 118 is coupled to driver 114 at its forward end and projects through the body for manual external turning at its other end. Detent plate 118 of assembly 116 includes forward detents to accommodate detent pin 119 in four different valve positions. In operation, face plate 112 is stationary while valve seat 111 rotates.

Plate 112 includes inlet and outlet purge ports 120 and 121 which align with an annular sealing groove 124 of valve seat 111. Face plate 112 also includes a central port tube 122 for either ingress or egress of gas and a series of five concentric port tubes 123 displaced radially from port 122 and mating with C shaped groove 68 in valve seat 111. A switching groove 126 is provided with one end facing port 122 and the other end at the same radial distance therefrom as C shaped groove 68. Detent plate 118 prevents valve seat 111 to be operatively positioned with groove 126 communicating with one of ports 123. Such positioning of valve seat 111 is controlled by a rotation of plate 118 to one of the four detent positions with a corresponding rotation of the valve seat. When gas 7 flows into port 122 it continues along groove 126 and out the selected one of ports 123. correspondingly, gas could flow into the same port 123 and out port 122. At the same time, the remaining four ports are interconnected via C ring 68 for continuous flow through all of the four port.

Sealing ring 124 includes a flowing sealing gas, which is preferably a carrier gas, and functions to prevent surrounding gases from leaking from a point external to the valve face into the gas streams flowing through the valve. This ring is particularly important since the detector 43 is highly sensitive to such extraneous gases.

It is apparent from the foregoing that an effective means has been provided for the multiple determination of a permeation coeflicient through a single membrane or through a series of different membranes in a rapid sequential manner. While one of the multiple cells is selected for determining permeability by sending carrier gas through the downstream side thereof through a detector, the remaining cells are continually purged of extraneous gases therefrom. By an appropriate switching of valves, one of the other cells may be employed for the determination of permeability in a rapid manner. Care is taken in the above described system to prevent any leakage from the surroundings which would interfere with the permeation detection system.

What is claimed is:

1. In a method for determining the permeability of membranes using first and second permeability cells, the steps of placing the membrane between the first and second cells to partition each cell into an upstream and downstream zone, continuously feeding permeant gas to said first upstream zone, directing permeant gas from said first upstream zone to said second upstream zone, venting said permeant gas from said second upstream zone, directing carrier gas to a selected one of said first or second downstream zones, removing from said selected downstream zone the carrier gas and permeant gas diffusing from the corresponding upstream zone, detecting the relative amount of permeant in the carrier gas, continuously directing purge gas through the one of said first or second downstream zones non-selected for said carrier gas direction, and venting said purge gas at atmospheric pressure, switching the flow through the selected and non-selected downstream zones, so that purge gas flows through the formerly selected zone and carrier gas is directed through the formerly r10n-selected zone to sweep permeant gas therefrom for detection.

2. A method as in claim 1 wherein the carrier and purge gas are of the same type.

3. A method as in claim 1 wherein the detection is performed by comparison of thermal properties of the permeant-carrier gas mixture to the properties of the carrier gas alone as a reference.

4. A method as in claim 1 including prior to feeding permeant to said first upstream zone the step of passing carrier gas through said first and second upstream zones and simultaneously passing the same carrier gas into a detector zone to set a reference base line for comparison with the gas mixture flowing from the permeation cell.

5. In an apparatus for determining permeation rates through a membrane, first and second permeation cells having upstream and downstream compartments adapted to be separated by the membrane retained therebetween, a conduit for permeant gas flow into said first cell upstream compartment, means forming a passage between said first and second upstream compartments, venting means for said latter compartment, permeant detector means, means incorporating first valve control means having a first operating position serving, when selected, to direct carrier gas into said first cell downstream compartment, and a second operating position serving, when selected, to direct the same gas to said second cell downstream compartment, means incorporating second valve control means operatively associated with said first valve control means serving to direct the carrier and diffused permeant gas from the selected first or second downstream compartment through said detector means.

6. An apparatus as in claim 5 wherein said first and second valve means are operatively coupled to direct a purge gas through the non-selected downstream compartment for removing extraneous volatile gases to the atmosphere.

7. An apparatus as in claim 5 wherein said first and second control valve means are of the rotary type and include sealing means formed of an annular groove and around the valve switching ports and a continuous stream of carrier gas flowing therein.

8. An apparatus as in claim 5 wherein said first and second valve means cooperate to form a setting whereby carrier gas is directed through said first and second valves and said detector means without flowing through any of the downstream compartments.

9. An apparatus as in claim 5 wherein said membrane is retained and sealed between said upstream and downstream compartments by means including spring means urging the mating wall of one of said compartments toward the other compartment mounted on a spring floated base.

References Cited UNITED STATES PATENTS 3,301,043 1/1967 Lyssy 7338 3,431,772 3/1969 Sunner et al. 7338 3,498,110 3/1970 Brun 73-38 LOUIS R. PRINCE, Primary Examiner W. A. HENRY II, Assistant Examiner 

